Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
Description of the Related Art
Display elements (pixel circuit) of an active matrix type organic EL display apparatus include organic EL elements and thin-film transistors (TFT). The current flowing through an organic El element is controlled by controlling the voltage to be applied to the display element, whereby emission brightness of the organic EL element is controlled. FIG. 9 shows a diagram depicting an example of the relationship between the voltage to be inputted to a display element (input voltage) and a emission brightness of the display element. FIG. 9 also shows a circuit diagram of the display element. The emission amount of the organic El element is approximately in proportion to the current flowing through the organic EL element. Therefore the relationship between the input voltage of the display element and the emission brightness is similar to the V-I characteristic of the TFT. In concrete terms, as shown in FIG. 9, the emission brightness of the display element rises from the vicinity of the threshold voltage Vth of TFT. Hence, if the electric characteristics (e.g. threshold voltage Vth) of the TFT disperse among the display elements, the relationship between the input voltage and the emission brightness disperses among the display elements, and brightness unevenness is generated in the display image (image displayed on the screen). Dispersion of characteristics of the display elements, such as the electric characteristics of TFT, is generated, for example, due to manufacturing problems of the display elements. The characteristics of the display elements also change by a change in ambient temperature of the display elements and deterioration due to aging of the display elements. Therefore dispersion of characteristics of the display elements is also generated by a change in ambient temperature of the display elements and deterioration due to aging of the display elements.
Prior arts to solve these problems are disclosed, for example, in Japanese Patent Application Laid-open No. 2005-345722, Japanese Patent Application Laid-open No. 2005-284172, and Japanese Patent Application Laid-open No. 2001-222257.
Japanese Patent Application Laid-open No. 2005-345722 discloses a technique of disposing a boot strap function and a Vth cancellation function in a display element (pixel circuit) to correct the V-I characteristics of the TFT in the circuit before the light emitting period of the display element.
Japanese Patent Application Laid-open No. 2005-284172 discloses a technique of preparing a gain correction value and an offset correction value to correct the dispersion of the threshold voltage Vth of the TFT and dispersion of the inclination of the V-I characteristics of the TFT in advance, and correcting the brightness of the image data using these correction values.
Japanese Patent Application Laid-open No. 2001-222257 discloses a technique of controlling the emission brightness without using a sub-threshold region where the dispersion of the V-I characteristics of the TFT is large. In concrete terms, a technique of controlling the emission amount of an organic El element by time-division is disclosed.
As shown in FIG. 9, the V-I characteristics of the TFT that supplies current to the organic EL element change at threshold voltage Vth as a turning point. In the V-I characteristics in a range of the input voltage which is less than the threshold voltage Vth (sub-threshold region), the current exponentially changes with respect to the input voltage. Therefore in the range of the input voltage which is less than the threshold voltage Vth, it is difficult to accurately control the current, and the brightness unevenness may be generated when images are displayed at a very low brightness when the input voltage is less than the threshold voltage Vth.
However, in the technique disclosed in Japanese Patent Application Laid-open No. 2005-345722 and Japanese Patent Application Laid-open No. 2005-284172, the V-I characteristics in the range of the input voltage which is not less than the threshold voltage Vth can be corrected, but the V-I characteristics in the range of the input voltage which is less than the threshold voltage cannot be corrected. In other words, in the case of the techniques disclosed in Japanese Patent Application Laid-open No. 2005-345722 and Japanese Patent Application Laid-open No. 2005-284172, brightness unevenness generated when display brightness is very low cannot be corrected.
Further, In the case of the technique disclosed in Japanese Patent Application Laid-open No. 2001-222257, the emission amount is controlled by time-division, hence the image quality of the display image deteriorates when a moving image is displayed. For example, when a moving image is displayed, such a problem as false contour (pseudo-contour) is generated in the displayed image.